unifDAG: Uniform Sampling of Directed Acyclic Graphs

Uniform sampling of Directed Acyclic Graphs (DAG) using exact
enumeration by relating each DAG to a sequence of outpoints (nodes with no
incoming edges) and then to a composition of integers as suggested by
Kuipers, J. and Moffa, G. (2015) <doi:10.1007/s11222-013-9428-y>.